Images may be formed on the surface of photoconductive materials by electrostatic processes. The electrophotographic imaging process involves producing a uniform electrostatic charge on a photoconductor, exposing it to a light and shadow image to dissipate the charge on the area, and then developing the resulting electrostatic latent image by depositing a finely divided electroscopic toner material. The toner material, or “toner” is electrically attracted to those areas of the photoreceptor which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. The developed image would then be transferred to a substrate such as paper, and permanently affixed to the substrate by heat, pressure, solvent or over-coating treatment. In the case of obtaining a multicolor image, such as making a colored photocopy, an original is first subjected to exposure through a color separation filter, and the process is then repeated by the use of color toners of yellow, magenta, cyan, and black, thereby forming a color image.
Toner materials may consist of binder resins, waxes or polyolefins, charge control agents, flow agents, pigments and/or dyes, and other additives. In a system in which a two-component developer is used, the toner particles may be employed in a mixture with solid carrier particles such as glass beads, iron powder or ferrite powder. The composition of toner and carrier is selected so that as a result of contact friction, the toner may experience a polarity reversal with respect to charge on the photoconductor layer. As a result of contact friction, the carrier electrostatically (triboelectrically) attracts the toner to its particle surfaces. This results in the transport of toner through a developing assembly and the feeding of the toner upon the photoconductive layer.
Dyes and pigments are suitable for some color toner applications. Such dyes and pigments may include organic dyes and pigments having relatively high tinting strength, good transparency, good thermal stability, and acceptable resin compatibility. Pigments may include: quinacridone, lithol rubine, rhodamine pigments for magenta, phthalocyanine blue pigments for cyan, and diarylide yellow pigments for yellow, among others.
There are problems and drawbacks when using pigments in toners. For example, a toner comprising an organic pigment that is insoluble in the resin may cause an undesirable decrease in the transparency or hue variation in the color of transmitted light. Improvements have been made to address this problem, including processes disclosed in U.S. Pat. Nos. 6,153,345 and 5,437,949. However, the use of pigments in toners to achieve color reproductions continues to represent a significant challenge in the toner manufacturing industry.
One advantage of selecting organic dyes instead of pigments for color toners is that dyes may exhibit increased color fidelity. This may be due to the degree to which the dyes may be molecularly dispersed in a binder resin. To obtain a homogeneous dispersion, it may be necessary to build into such molecules substituting groups for enhancing compatibility with the resin. Unless the dye molecules are substantially fully compatible with the resins, these molecules may have a tendency to aggregate over time when subjected to heat, pressure and humidity. This may result in a loss of color fidelity. The low molecular weight of the dye molecules may cause high mobility of the dyes in the resin, resulting in undesirable bleeding of the dyes. Thus, the use of dyes in toner applications also may provide drawbacks and disadvantages.
There is continuing long felt need in the industry for toner compositions and toner processes that are capable of achieving a balance of relatively high resolution, low cost, reduced environmental risk, low levels of migration, reduced levels of bleeding, and more environmentally sound manufacturing processes. This invention addresses these needs in the industry.